This invention relates to interferometric distance measurements using optical signals, and more particularly to a method and apparatus for enhancing precision of such measurement by virtually eliminating polarization leakage problems in an interferometer and by compensating for undesirable variations in the system, including variations caused by air turbulence.
Interferometers are currently utilized for distance and position measurement in a variety of applications including lithographic integrated-circuit wafer production, measuring satellite position in global position sensing (GPS) systems, and measuring distance to detectors in earthquake detection systems. In all of these systems, undesirable perturbations of the interferometer optical signals due to natural phenomena or other variations in system parameters can result in small errors in the distances being measured.
One particular source of errors is air turbulence in the measurement space through which an interferometer optical signal, such as a light beam, passes. Turbulence is defined for purposes of this invention as variation in local density of the air in the measurement space. Such air density variations can result from a number of factors, including local temperature variations and air movement. Since the refractive index of the air through which the optical signal passes varies slightly with the density of the air, such turbulence can cause small errors in the distance measurements, as the distance measurement is a function of the wavelength of the optical signal and the refractive index of the air.
Existing high quality single-wavelength interferometers can measure an optical path-length, for example a path-length used in lithography as a measure of stage position, with a theoretical precision on the order of 1 nm or better. However, turbulence of the air in the interferometer optical signal path typically contributes variations of 10-30 nm to the measured path-length during the typical time period in which an integrated-circuit wafer is exposed.
Since such single-wavelength interferometers cannot distinguish between path-length changes due to this air turbulence and those due to stage motion, air turbulence has the effect of degrading the precision of these interferometers to a point where they are marginally capable of supporting 0.25 xcexcm-design-rule lithography. Hence, 0.1 xcexcm-design-rule lithography and below, which are becoming increasingly important in the industry, present significant challenges to the accuracy and precision of single-wavelength interferometers. As a result, under typical wafer production conditions, the overlay precision of single-wavelength interferometers is limited by air turbulence to approximately 10-30 nm, which is an unacceptably large imprecision for 0.1 xcexcm-design-rule lithography.
One solution which has been proposed to overcome the air turbulence problem is for two interferometers employing light beams having significantly different wavelengths (or frequencies) to share a common measurement path. The optical path-length of the measurement path xe2x80x9cseenxe2x80x9d by each light beam will differ because the refractive index of air is a function of wavelength. This small but significant difference can be used to directly determine the optical xe2x80x9cthicknessxe2x80x9d of the air path, allowing a correction for turbulence to be made.
While conventional interferometer systems utilizing two light beams have purportedly improved measurement precision by correcting for air turbulence, in general such systems are not readily integrated with existing single-wavelength interferometers to improve the precision of single-wavelength interferometers. Additionally, the precision of conventional multi-wavelength interferometers is limited by factors other than air turbulence, which render such interferometers marginally capable of meeting the stringent requirements of very high precision applications, for example, 0.1 xcexcm-design-rule lithography, as discussed above.
In view of the foregoing, there is a need for an improved technique to eliminate or compensate for various potential overlay error sources for position lithography applications, particularly in 0.1 xcexcm-design-rule lithography. In general, an improved precision interferometric distance measurement technique is desirable in order to eliminate or compensate for various sources of measurement error, including air turbulence, in these and other interferometer applications.
For example, an additional problem with conventional single or multi-wavelength interferometers, unrelated to the problem of air turbulence, is polarization leakage or optical nonlinearity. This problem arises because the optical signal splitters typically employed in conventional interferometers to separate the optical signal into two polarized components are imperfect, and therefore some percentage of the optical signal polarized to pass through one of two optical paths is, in fact, in the other of the two optical paths. The nonlinearity of measurements resulting from this xe2x80x9ccrosstalkxe2x80x9d error presents a problematic limit to precision in conventional interferometers, and a solution to this problem would provide an advantage in further enhancing the precision of interferometric distance or position measurements.
It is additionally desirable that an improved interferometer system be achromatic (function equivalently for a wide range of wavelengths), that any precision enhancement and error correction mechanism be compatible with conventional interferometers and require minimal additional space (for purposes of xe2x80x9cretrofittingxe2x80x9d an existing interferometer with the improved interferometer system), and that the technique utilized for error correction be easily adaptable to perform a xe2x80x9cbaselinexe2x80x9d interferometer distance measuring function, in addition to the error correction function, to provide an enhanced precision distance measurement.
It is also desirable, particularly with respect to lithographic integrated-circuit wafer production applications, that all electrical signals and potential heat generating components be mounted remotely from an interferometer measurement head and from any lithographic components so that the measurement head for the system, which may be mounted to the lithographic stage, is completely passive and contains no thermal sources, thereby eliminating a potential limitation on measurement precision. Further, it is desirable that the passive measurement head itself be extremely insensitive to ambient temperature variations, thereby overcoming temperature drift problems that have also been a source of error in conventional interferometers. Finally, the measurement head should be rugged and inherently insensitive to motion and vibration.
In accordance with the above, the present invention provides a method and apparatus for enhancing the precision of interferometric distance and position measurements.
One example of the present invention is an inexpensive compact two-wavelength interferometer using an analog radio-frequency (RF) heterodyne-mixing signal processing technique, alone or in combination with digital signal processing techniques, and a novel measurement head design, which can significantly reduce measurement errors, such as those due to air turbulence, to less than 1 nm by simultaneously measuring an optical path-length at two different wavelengths. The two different wavelengths may be harmonically related, or may have an arbitrary wavelength relationship.
One aspect of the present invention is an achromatic interferometer design that allows complete integration of an interferometer according to the present invention with a conventional baseline measurement system. The design permits a non-invasive retrofit requiring no modification of the baseline system and allows all optical signals to share a single, compact measurement head.
Another aspect of the present invention is an interferometer system that virtually eliminates the optical nonlinearity xe2x80x9ccrosstalkxe2x80x9d problem due to polarization leakage, common to conventional interferometers, thereby improving an intrinsic (no-turbulence) precision of a baseline interferometer according to the invention to better than 1 nm. This is accomplished primarily by a unique measurement head design which allows a measurement and a reference optical signal to travel distinct paths throughout the system, for example, by spatially different paths, different path directions, or different polarizations.
Another aspect of the present invention is an achromatic interferometer system that includes a passive interferometer measurement head with no thermal sources that could limit the measurement precision.
Another aspect of the present invention is an interferometer measurement head which is extremely insensitive to temperature variations.
Another aspect of the present invention is a monolithic interferometer measurement head design which makes the head rugged and inherently insensitive to motion and vibration.
Another example of the present invention is a single-wavelength interferometer which is added to an existing conventional baseline system to allow for measurement precision enhancement by, for example, correcting for errors, such as those due to air turbulence, and minimizing measurement nonlinearity due to polarization leakage. The baseline optical signal can serve as one of the two-optical signals required to measure the optical path-length difference due to the wavelength-dependent refractive index of air, while simultaneously measuring absolute distance or position.
Another example of the present invention is an achromatic interferometer, using an analog radio-frequency (RF) heterodyne-mixing signal processing technique, alone or in combination with digital signal processing techniques, that can function as a stand-alone system, measuring absolute distance or position and simultaneously correcting for measurement errors, including those due to air turbulence.
Another example of the present invention is a multiple-axis positioning system which employs one two-wavelength interferometer of the present invention for each degree of freedom of movement, or positional axis. One optical signal source may be used for each wavelength required, regardless of the number of axes controlled by the positioning system. The respective optical signal source powers can be split amongst the respective interferometers required for each axis.
Another example of the present invention is an interferometer apparatus comprising an optical module to modulate at least first and second optical signals with at least first and second modulating signals, respectively, to provide at least first and second modulated optical signals. The interferometer apparatus also includes a measurement head optically coupled to the optical module and constructed and arranged to direct each modulated optical signal through a reference optical path and a measurement optical path. The optical module is constructed and arranged to optically process each modulated optical signal after each modulated optical signal has traveled through the reference and measurement optical paths. The optical module outputs at least two difference signals, each difference signal corresponding to a respective modulated optical signal and having a difference signal frequency derived from a respective modulating signal. Each difference signal represents a path-length difference between the reference and measurement optical paths of the respective modulated optical signal. The interferometer apparatus may also include a signal processor to process the difference signals to output a correction signal based on a beat frequency derived from the first and second modulating signals. The correction signal represents a path-length difference between the measurement optical paths of the first and second modulated optical signals.
Another example of an interferometer apparatus according to the invention comprises at least one optical signal and a splitter to divide each optical signal into a reference component and a measurement component, to form a reference component set and a measurement component set. The interferometer apparatus also includes a plurality of optical elements arranged to direct the reference component set through a reference optical path and the measurement component set through a measurement optical path. The reference optical path is distinct from the measurement optical path throughout the apparatus. The apparatus further includes at least one detector to detect the reference and measurement components of each optical signal after the components have traveled through the reference and measurement optical paths, respectively. The detector outputs at least one difference signal based on a path-length difference between the reference and measurement optical paths, wherein each difference signal corresponds to a respective optical signal. The plurality of optical elements may be arranged such that the reference and measurement component sets do not travel in a like direction on a shared optical path. The plurality of optical elements may also include a measurement head, wherein the plurality of optical elements are arranged such that the reference and measurement component sets arrive at the measurement head on spatially separated paths.
Other objects, novel features and advantages of the present invention will become apparent to those skilled in the art through the description of a preferred but not exclusive embodiment, claims, and accompanying drawings.